A need exists for polymer composites that are thermally conductive, yet electrically insulative. Polymers are electrical and thermal insulators, so thermally conductive fillers have to be added to provide thermal conductivity. High volume contents of filler are needed to achieve thermal conductivities suitable for efficient heat transport through a polymer composite. The use of electrically conductive fillers in such applications is limited to low volume contents, since electrical conductivity is achieved at significantly lower filler loadings than thermal conductivity. Therefore, typically so-called ceramic fillers with high thermal conductivities are added to these polymer compositions. The number of ceramic filler types that can currently be used for thermally conductive composites is relatively limited, since most ceramic fillers are thermally insulative or have a relatively low thermal conductivity. Consequently, ceramic fillers with a high thermal conductivity are typically expensive to use and the design freedom to control other properties, such as mechanicals, flow and heat stability, is limited. There is a need in the art for improved fillers or combinations thereof that achieve thermally conductive, yet electrically insulative properties of polymer composites. In addition, there is a need in the art for less expensive fillers.